More particularly, the invention relates to an imaging method using shear waves for observing a diffusing viscoelastic medium containing particles that reflect ultrasound compression waves, said method comprising:
a) an excitation step during which an elastic shear wave is generated in the viscoelastic medium;
b) an observation step during which the propagation of the shear wave is observed simultaneously at a multitude of points in an observation field in the viscoelastic medium, this observation step comprising the following substeps:                b1) causing an array of transducers that are controlled independently of one another to emit into the viscoelastic medium a succession of unfocused ultrasound compression wave shots at a rate of at least 500 shots per second; and        b2) causing sound signals received from the viscoelastic medium to be detected and recorded in real time, said signals comprising the echoes generated by the unfocused ultrasound compression wave interacting with the reflecting particles in said viscoelastic medium; and        
c) at least one processing step during which:                c1) the sound signals received successively from the viscoelastic medium during substep b2) are processed in order to determine successive propagation images of the shear wave; and        c2) at least one movement parameter of the viscoelastic medium is determined at different points of the observation field.        
This produces a “motion picture” clearly illustrating the propagation of the shear wave through the viscoelastic medium, which can make it possible to perform qualitative and/or quantitative analysis in order to identify zones having hardness that differs from the hardness of the remainder of the viscoelastic medium, or zones having relaxation time that differs from the relaxation time of the remainder of the viscoelastic medium.
Document WO-A-00/55616 describes an example of such a method, in which shear waves are generated at the surface of the viscoelastic medium. That method gives full satisfaction in particular when imaging zones situated relatively close to the surface of the viscoelastic medium. However that known method does not enable certain zones to be observed in the viscoelastic medium, and in particular:
zones that are sufficiently deep to be unreachable by shear waves generated at the surface (shear waves attenuate quickly); and
shadow zones that are masked by obstacles (in particular portions of a patient's skeleton or liquid zones such as liquid cysts) which impede the propagation of shear waves.
In addition, if the observation field is partially in a shadow zone, it can be necessary to move the shear wave generator device during an observation, which is tedious for the user.
Finally, the shear wave generator device is relatively heavy and complicates the apparatus.